x=null * Expected NULL Observed NULL ? NULL = x * Expected NULL Observed NULL ? NULL + x * Expected NULL Observed NULL ? -X && - (Logical null) * Expected NULL Observed Error => Bug ? 0-X y=3 y=null * Expected NULL Observed NULL ? Y= X && (numeric null) = (Logical null) * Expected NULL Observed NULL ? Y AND X && (Logical null) AND (numeric null) * Expected NULL Observed Error => Bug ? y - X && (numeric null) - (Logical null)Effect :